This invention relates to skid control systems for the brakes of vehicles, such as automobiles and the like; and particularly to systems which prevent an operator from losing control of a vehicle, due to locking of the wheels, by regulating the effectiveness of the brakes in dependence upon the road surface conditions, and regardless of the pedal effort, during an emergency stop.
The invention has particular, although not exclusive reference, to systems for regulating the pressure of brake fluids applied to the wheel cylinders of a brake.
A skid control system may be composed of a wheel velocity detector, a control unit and a pressure modulator for modulating the pressure of brake fluid. In such a skid control system, a deceleration is preset as a desired limit on the basis of road surface conditions which are indirectly detected from factors, such as changes in the wheel velocity. The control unit compares the preset deceleration with the actual wheel deceleration. The latter is derived from the wheel velocity as detected by the wheel velocity detector. The comparator issues pressure reduction signals to the pressure modulator when the wheel deceleration exceeds the preset deceleration. The pressure modulator responds by reducing the pressure of the brake fluid.
Known methods for determining the actual wheel deceleration employ a differentiation circuit composed of resistors and capacitors. The differential value of wheel velocity is then used as the wheel deceleration.
It is difficult to obtain accurate indications of actual wheel decelerations by means of differentiation circuits, because at the low frequency at which the velocity varies, namely on the order of several Hertz, it is difficult to obtain exact differential values. Moreover, the differential signal obtained is subject to distortion by noise. This results in skid control systems with unstable performance.
Also, skid control systems are required to perform their braking functions so as to achieve minimum stopping distances while preventing the wheels from locking. Under such circumstances it is desirable that braking be accomplished when the coefficient of friction between the wheels and the road surface is maximum. Hitherto it has been assumed that the point of maximum coefficient of friction is usually in the range of 0.15 - 0.2 of the wheel slip ratio. Consequently, systems have been used wherein a constant slip ratio control falling within this range is set and utilized to regulate a pressure modulator. In such systems, if the slip ratio at which the coefficient of friction is maximum, varies because of changes in the road surface or because of changes in the condition of the road surface or because of changes in the shapes of tires or for any number of reasons, skid control is reduced.
An object of this invention is to improve skid control systems.
Another object of this invention is to obviate the disadvantages of previous skid control systems.
Still another object of the invention is to render skid control systems responsive to a number of variables, such as the kind of road surface and its condition.
Yet another object of the invention is to adjust the skid control system continuously for various road conditions and other variables, yet another feature of the invention is to adjust skid control systems in a manner most appropriate for braking a vehicle with a slip ratio having the maximum coefficient of friction.